git hook run: add an --ignore-missing flag

For certain one-shot hooks we'd like to optimistically run them, and
not complain if they don't exist.

This was already supported by the underlying hook.c library, but had
not been exposed via "git hook run". The command version of this will
be used by send-email in a subsequent commit.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/hook.c b/builtin/hook.c
index 9b67ff5..54e5c6e 100644
--- a/builtin/hook.c
+++ b/builtin/hook.c
@@ -7,7 +7,7 @@
 #include "strvec.h"
 
 #define BUILTIN_HOOK_RUN_USAGE \
-	N_("git hook run <hook-name> [-- <hook-args>]")
+	N_("git hook run [--ignore-missing] <hook-name> [-- <hook-args>]")
 
 static const char * const builtin_hook_usage[] = {
 	BUILTIN_HOOK_RUN_USAGE,
@@ -23,8 +23,11 @@ static int run(int argc, const char **argv, const char *prefix)
 {
 	int i;
 	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
+	int ignore_missing = 0;
 	const char *hook_name;
 	struct option run_options[] = {
+		OPT_BOOL(0, "ignore-missing", &ignore_missing,
+			 N_("silently ignore missing requested <hook-name>")),
 		OPT_END(),
 	};
 	int ret;
@@ -52,7 +55,8 @@ static int run(int argc, const char **argv, const char *prefix)
 	git_config(git_default_config, NULL);
 
 	hook_name = argv[0];
-	opt.error_if_missing = 1;
+	if (!ignore_missing)
+		opt.error_if_missing = 1;
 	ret = run_hooks_opt(hook_name, &opt);
 	if (ret < 0) /* error() return */
 		ret = 1;